The overarching theme of the current program project grant is to define how the innate immune system informs the adaptive immune system in the intestine and how miscommunication in this interaction results in the development of IBD. Our currently funded grant has a strong emphasis on the role of the lEC in regulating innate and adaptive Immune responses in health and disease. While the epithelial cell will continue to be a prominent focus in this renewal, the data generated thus far have allowed the projects to evolve in the direction of studying the regulation of Th17 responses, a key cell type involved in mucosal inflammation. All projects have aims that involve the use of mouse and human models allowing the investigators to make observations in human tissues that can then be modeled in the mouse and vice versa (results obtained through the study of animal models can be validated by studying human tissues). In the first project the focus is on assessing the role of a newly defined population that expresses both FoxP3 and IL-17 in Crohn's disease tissues and may represent the previously undescribed interface between Tregs and Th17 cells. The second project focuses on the transcription factor associated with Th17 development, ROR?t, by analyzing the contributions of chemokines in driving these cells to the gut (an extension of the previous granting period) and the role that IL23 plays in the development of Th17 mediated inflammation (taking advantage of murine models developed by the PI). The third project also reflects the evolution of studies from the first granting period by studying the role of the transcription facto IRF8 (ICSBP) on Th17 differentiation (acting as a negative regulator of IL17 production and ROR?t function) and its role in IBD pathogenesis. The fourth project is new to this PPG but takes advantage of the growth of mucosal immunology at Mount Sinai. Dr. Julie Blander studies the role of TLR signaling in Th17 responses in vitro and in vivo using infected vs. uninfected apoptic cells including an infection model (C. rodentium). Her studies fit perfectly with the other projects in our program and her participation in the group provides her access to unique resources to study murine and human IBD. At the same time, she provides models (C. rodentium) that will be helpful for the other PPG members. Overall the focus and interactive nature of the program (20 joint publications in 4 years, 3 additional NIH grants on related areas) provide a solid basis for a productive outcome.